Archer passive shield not blocking projectiles

i was in FFA and had a team of archers going and only one archer was against us, i turned around looked up and crouched with my passive shield on my back and the other archer shot right through it and killed me. i know i had the shield facing where he couldnt get around it because i switch to third person to make sure its lined up. we were on ruins and i was on the pillar u can jump to and he was in the hut arcoss the river.